In electroless plating of copper with silver, a copper metal is immersed in a water solution of silver cyanide. Immersion of the copper metal coats the red copper surface with a bright silver plate. The cyanide bath is prepared by salting silver nitrate in water to precipitate silver chloride. A cyanide salt is then added to dissolve the precipitated silver chloride into a silver cyanide complex which is soluble in water.
This process of plating is also known as autocatalytic, charge exchange, and substitutional plating. Electroless plating does not require battery power to drive metal deposition. Electroplating does require the complication of an added battery.
Aqueous silver plating solutions, even with cyanide addition, are unstable. In time, the solutions darken from exposure to light and airborne dust. Also, because water is a protic solvent, hydrogen is formed as a byproduct of the plating reaction to dull and crack the brightness of the silver plate.
Bonding of a silver film to a base copper wire by aqueous cyanide plating is also subject to cracking and rusting, commonly called “the red plague.” The plague is caused by hydrogen, copper and silver reacting with water to form red and black spots during the electroless plating process.
Now, that local and federal environmental laws have restricted use of silver cyanide, there is an even greater need for an improved cyanide free substitute.
Examples of aqueous electroless plating include:
(a) U.S. Pat. No. 5,322,553 in which the plating process is one step, comprising immersing nickel metal in:
hot water as the solvent;
silver sulfite and silver thiosulfate as both the supplier of the silver salt and reducer of the silver salt to a bright metal;
sodium EDTA as the silver complexing agent.
(b) U.S. Pat. No. 6,387,542 in which the plating process is one step, comprising immersing metal and non-metal substrates in:
boiling water as the solvent;
silver nitrate as the silver salt;
ammonia as the silver complexing agent;
hydrazine as the reducer of the silver salt to a bright metal.
(c) U.S. Pat. No. 6,387,542 revealing electroless bright silver deposition on a steel substrate in a multiple step process with an aqueous solution including a silver salt, an ammonium salt to complex the silver salt, and a hydrazine salt to reduce the silver salt to a bright metal.
An example of a waterless electroless plating method includes:
(d) U.S. Pat. No. 8,298,325 revealing electroless copper deposition on a non-metallic substrate sensitized with two non-aqueous solutions comprising copper and cobalt and an ammonium salt complexing component. A second electroplating step is not required to form the bright metal plate, but the reduction of the copper salt to a bright copper plate is a multiple step process.
An example of waterless electroless plating using a polar aprotic solvent includes:
(e) U.S. Pat. No. 3,963,841 revealing electroless metal deposition on a non-metallic substrate sensitized with a non-aqueous palladium salt in dimethyl sulfoxide as a first step. A second aqueous electroplating step is required to form a bright silver plate.
Examples of silver diammine complexes include:
(f) U.S. Pat. No. 4,197,275 reveals recovery of silver bromide from waste photographic film with ammonium bromide dissolved in non-aqueous dimethyl sulfoxide without claiming the process as a plating aid.
(g) U.S. Pat. No. 5,514,261 reveals a silver diammine complexing ion for cyanide free aqueous electroplating of silver in a multiple step process.
Electroplating pens are common in the jewelry industry for coating yellow gold with a bright rhodium plate. These pens require the complication of a battery. Petitioner is aware of a need for an electroless ink useful for one step silver plating at room temperature by a pen means.